Radiofrequency (RF) ablation is a medical procedure which can be used to treat some types of rapid heart beating including conditions such as supraventricular tachyarrhythmias. Ablation can be used to treat a wide variety of tachycardias, which can involve heart tissues in the upper chambers (atria), also called supraventricular, or the lower chambers (ventricles), called ventricular tachycardias (SVT or VT). In some cases, a small number of cells (a “focus”) start firing rapidly and ablation can eliminate the focus. In other types of tachycardia, an electrical circuit exists within which the electrical signal travels more or less in a circle (“reentry”). Many cases of SVT and VT are due to reentry. A special type of SVT, called atrial fibrillation, is characterized by extremely fast impulses in the atrium (up to 600/min), for which ablation can be used either to decrease the number of impulses getting to the ventricles or in some cases to locate and ablate the area(s) from which the fibrillation starts. (Circulation. 2002; 106:e203.)
An example of an ablation procedure is as follows: A physician guides a catheter with an electrode at its tip to the area of heart muscle where there's an accessory (extra) pathway. The catheter is guided with real-time, moving X-rays (fluoroscopy) displayed on a video screen. The procedure helps the doctor place the catheter at the exact site inside the heart where cells give off the electrical signals that stimulate the abnormal heart rhythm. Then a mild, radiofrequency energy (similar to microwave heat) is transmitted to the pathway. This destroys carefully selected heart muscle cells in a very small area (about ⅕ of an inch). That stops the area from conducting the extra impulses that caused the rapid heartbeats.
Radiofrequency current can be alternating current that is delivered at cycle lengths of 300 to 750 kHz when used for catheter ablation. It causes resistive heating of the tissue in contact with the electrode. Because the degree of tissue heating is inversely proportional to the radius to the fourth power, the lesions created by radiofrequency energy are small. Typical ablation catheters, which are 2.2 mm in diameter (7 French) and have a distal electrode 4 mm long, create lesions approximately 5 to 6 mm in diameter and 2 to 3 mm deep. Larger lesions are possible with larger electrodes or saline-irrigated ablation catheters. Although electrical injury may be a contributing factor, the primary mechanism of tissue destruction by radiofrequency current is thermal injury. Irreversible tissue destruction can require a tissue temperature of approximately 50° C. In most ablation procedures, the power output of the radiofrequency generator is adjusted manually or automatically to achieve a temperature of 60 to 75° C. at the electrode-tissue interface. If the temperature at the electrode-tissue interface reaches about 100° C., coagulated plasma and desiccated tissue may form on the electrode, preventing effective delivery of the current, predisposing the patient to thromboembolic complications, and necessitating the removal of the catheter so that the coagulated material can be wiped off the electrode. The acute lesion created by radiofrequency can consist of a central zone of coagulation necrosis surrounded by a zone of hemorrhage and inflammation. Chronic lesions are often characterized by coagulation necrosis and have a discrete border. Changes that occur in the border zone can explain why arrhythmias may recur several days to several weeks after apparently successful ablation. The arrhythmia may recur if the target tissue is in the zone bordering a lesion instead of in the central area of necrosis and if the inflammation resolves without residual necrosis. Conversely, the site of origin of an arrhythmia that has not been successfully ablated may later become permanently nonfunctional if it is within the border zone of a lesion and if microvascular injury and inflammation within this zone result in progressive necrosis. (New England J of Medicine, Volume 340:534-544, Feb. 18, 1999).